Many alternative energy sources deliver an approximately constant electrical power over short periods of time which can vary over longer durations based on operating conditions, such as solar energy availability for photovoltaic cells or fuel-supply-based output power from fuel cells. Alternative energy sources include solar panels and fuel cells, which produce power with direct-current (DC), and wind or other rotating or reciprocating generation systems that usually produce power at variable frequency then use a rectifier to produce DC power for power conditioning. This DC electrical power is processed through a power conditioner, generally for conversion to sinusoidal alternating-current (AC) power at fixed frequency, either for delivery to a conventional electricity grid or directly to a load for “off-grid” applications. Typical conversion systems utilized in residences or small businesses generate single-phase AC power, with sinusoidal voltage and current at a fixed nominal frequency.
A basic electrical property of a single-phase AC power system is that the energy flow includes both an average power portion that delivers useful energy from the energy source to the load and a double-frequency portion that flows back and forth between the load and the source. The double-frequency portion represents undesirable ripple power that can compromise performance of the DC power source. Power conditioners for alternative energy systems preferably draw power from the source at the input of the power conditioner without ripple, and in turn deliver both average power and the double-frequency ripple power to the AC load at the electrical output. The conservation of energy principle requires that there exists some physical effect inside the power conditioner to manage this double-frequency ripple power.
Typically, power conditioners used for DC energy sources such as photovoltaic and fuel cells are configured as DC-to-AC converters, and are often referred to as inverters. Practical inverters for single-phase alternative energy systems include filters to manage double-frequency ripple power. The filters are configured to allow double-frequency ripple power to flow at the AC output of the inverter, while minimizing or preventing similar ripple power from flowing back to the DC energy source or otherwise being imposed on the DC energy source.
To manage double-frequency ripple power, energy needs to be stored and delivered at twice the AC frequency. The electrical components needed to store the needed energy are generally large and are well known to be the least reliable components in power inverters. A typical energy storage component is a large electrolytic capacitor, which has well-known failure and wear-out modes that prevent reliable operation over a long lifespan. A typical electrolytic capacitor might have a nameplate rating of 2,000 hours (less than 7 months in a typical solar inverter application) of operation at its maximum temperature and voltage. Since inverters need to operate for longer durations, expensive derating methods, such as those based on the Arrhenius equation, typically limit the operating temperature and operating voltage in order to extend the device lifetime. Still, today's state-of-the-art derating and production methods, well known to one skilled in the art, support inverter warranties of only about 5 years, as observed in the marketplace. Longer warranties usually assume that the electrolytic capacitors will be replaced during the inverter lifetime.
Solar inverters, when rated for outdoor use and co-packaged with the solar panels, often operate at elevated temperatures which accelerate the failure modes and shortens the lifetime of electrolytic capacitors mounted in the inverter. Even though the inverters only run during the part of each day the sun shines, thermal stress prevents the inverters from lasting twenty-five years or more—in contrast to solar panels which often have warranties of 25 or more years. Indeed, manufacturers of inverters for solar power have stated categorically that 20 year inverter life is not possible, largely because of this component issue.
As electrolytic capacitors are well known to be the most significant limitation to power inverter reliability, expectations for poor reliability are evident in the market place: high-power inverters are designed to have replaceable (field serviceable) electrolytic capacitors, new lower-power inverter designs feature easily removable inverter electronics to facilitate repair or replacement, and service contracts are routinely sold with inverters in some markets. While electrolytic capacitors have many failure modes, a major wear-out mechanism is that ripple current causes self-heating which in turn reduces life. Since self-heating is from within, the actual core temperature is higher than the ambient, limiting the effectiveness of active cooling techniques. A typical solution is to use capacitors rated at 105° C. in place of more common 85° C. capacitors but they add 20% to 50% more to the cost and are an incomplete solution.
Many commercially available inverters manage the double-frequency ripple power by using passive filtering in the form of an electrolytic capacitor which is applied at a DC bus where the double-frequency power term translates into ripple on the capacitor. This passive filtering arrangement requires a large capacitance value to filter the double-frequency power, since the necessary energy exchange needs to be supported without imposing significant voltage ripple. Further, since the capacitor maintains a relatively constant voltage, the capacitor current needs to flow at the double-frequency.
In the research community, active filtering circuits are being explored as a more effective alternative to the passive methods. In active filter approaches, ripple current is supplied through a separate power converter. In one common approach, a capacitor is used to maintain a relatively fixed voltage at a separate location within the power conditioner, a so called “internal DC bus.” A controller injects a compensating current from this capacitor into the inverter circuit to cancel out the double-frequency ripple power. Since the voltage is held approximately fixed, the compensating current is injected at double the AC line frequency. A typical example injects this compensating current at the terminals of photovoltaic (PV) array.
The active filter method provides two advantages over passive approaches: (1) the capacitor voltage can be higher than the voltage of the PV array, increasing the available energy, and (2) more ripple power can be tolerated on the capacitor than on the PV array. The required capacitance has been shown to be:
  C  =      P          2      ⁢      π      ⁢                          ⁢      fV      ⁢                          ⁢      Δ      ⁢                          ⁢      V      where P is the average output power, f is the fundamental grid frequency, V is the average capacitor voltage, and ΔV is the allowed peak-to-peak ripple voltage. This method leads to significant capacitance reduction over passive filter approaches. For example, a 1 kW inverter with a 200V capacitor bus voltage, requires only about 440 μF if 15% ripple is allowed on the capacitor. This is a factor of almost 20 reduction over passive filtering, but is still high enough to require large electrolytic capacitors.
Therefore, a need exists for a control technique that minimizes double-frequency ripple power in power conditioners, removes the need for large electrolytic capacitors, increases the lifespan of power conditioners, reduces cost and overcomes other problems previously experienced. These and other needs will become apparent to those of skill in the art after reading the present specification.